Power Over Ethernet (POE), which is outlined in IEEE Std 802.3™-2005 clause 33 (the POE standard), refers to a technique for delivering power and data to an electronic device via Ethernet cabling. In a POE system, a power source (or sourcing) equipment (PSE) device provides a power supply via two of four wire pairs of an Ethernet cable to electronic devices, which may be referred to as powered devices or PDs. POE makes it possible to deliver power to attached powered devices without requiring each PD to have access to a power source. Such PDs may include voice over Internet protocol (VoIP) telephones, wireless routers, security devices, field devices to monitor process control parameters, data processors, and the like.
The POE standard specifies protocols for detecting a PD and for determining how much power to supply to the PD through the Ethernet cable. In particular, the PSE device performs a PD detection operation to determine whether the PD is attached before supplying power via the Ethernet cable. To perform the PD detection operation, the PSE device applies a DC voltage (within a range of 2.8 to 10 Volts DC) to one of the pairs of wires of the Ethernet cable and monitors a received current or a received voltage to detect a PD resistance within an expected range (e.g. between 19 and 26.5 K-ohms). The PSE device determines if a PD is connected to one of the ports of the PSE device using a Volt-Amp (VA) slope related to the PD's voltage/current signature. If the PSE device does not detect a valid resistance, the PSE device does not apply power to the Ethernet port. However, if the PSE device detects a valid signature (i.e., an expected resistance), the POE standard specifies that the PSE device may either supply a default power level to the PD or optionally perform a power classification operation to determine power requirements of the detected PD.
If the PSE device supports power classification, the PSE device may apply a classification voltage to the Ethernet port associated with the detected PD. Assuming that the PD supports classification, the PD produces a classification current for the device. The PSE determines the PD's power classification based on this classification current.
The POE Standard defines five classes having pre-determined power levels and defines an appropriate response for a PD to select its power level by providing a corresponding classification current. For example, a current draw of zero to four mA corresponds to class 0, which is also the default class for devices that do not support the optional POE classification. A current draw of 26 to 30 mA and of 36 to 44 mA corresponds to class 3 and class 4 devices, respectively. The POE standard specifies that the PSE provide a power supply of approximately 15.4 watts to devices of class 0, class 3 and class 4. A current draw of between 9 and 12 mA corresponds to a class 1 device, and the POE standard specifies that the PSE provide a power supply of up to approximately 4 watts to a class 1 device. A class 2 device corresponds to a current draw of 17 to 20 mA and requires the PSE to provide approximately 7 watts of power.
In 2009, the Institute of Electrical and Electronics Engineers or IEEE adopted an updated version of the POE Standard, which is referred to as the IEEE 802.3 at-2009 PoE standard, “POE+” or “POE plus”. POE plus provides an enhanced power classification that allows the PSE device to supply up to 25.5 watts of power, but does not comprehend a PSE device or PD using all four pairs of wires of the network cable for power.